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Conformational analysis of 2,2,2-trichloroethyl methyl ether by low-resolution microwave spectroscopy
Journal of Molecular Structure 550–551 (2000) 99–104 www.elsevier.nl/locate/molstruc
Conformational analysis of 2,2,2-trichloroethyl methyl ether by low-resolution microwave spectroscopy 夽 B.-Y. Liu, H. Mohamad, Y.-S. Li* The University of Memphis, Department of Chemistry, Memphis, TN 38152, USA Received 6 July 1999; accepted 9 November 1999
Abstract The low-resolution microwave spectra of 2,2,2-trichloroethyl methyl ether have been recorded from 26.5 to 39.0 GHz. From the spacing of the major bands, it is shown that the value of 2065.4 MHz for B ⫹ C is consistent with the presence of the gauche conformer. Another set of bands of approximately two-third the intensity of the major bands has also been observed. The value of 1996.5 MHz for B ⫹ C is consistent with the presence of the trans conformer. The microwave data indicate that the gauche conformer is the abundant one in the gas phase. Interpretation as to the dominance of the gauche conformer will be suggested. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Conformational analysis; 2,2,2-Trichloroethyl methyl ether; Microwave spectrum; Low resolution microwave spectrum
1. Introduction The 2,2,2-trichloroethyl methyl was first prepared in 1951 [1]. It was then synthesized as anesthetics [2,3]. Zanke et al. [4] reported that the compound has preventive and curative activities against Phytophthora infestants on tomato leaf disks. In an earlier study, we recorded the infrared and Raman spectra in the frequency region 4000–35 cm ⫺1, carried out a normal coordinate analysis, and assigned the vibrational modes for the molecule [5]. Additionally, we have identified two non-equivalent conformers in the study. From the variation of the relative intensities of the CCO bending vibrations with temperature in liquid, the enthalpy difference was
determined to be 116 ^ 11 cm ⫺1 with the trans conformer being more stable. In the present study, we have collected the lowresolution microwave spectrum of 2,2,2-trichloroethyl methyl ether. The basic information obtained from the analysis of the experimental rotational transition frequencies includes the molecular rotational constants, which are inversely proportional to the principal moments of inertia. The principal moments of inertia are closely related to the molecular structure. In general, non-equivalent rotational isomers have different principal moments of inertia and hence distinctly different rotational spectra. By comparing the calculated rotational constants to the observed ones, the conformation of a molecule can be identified.
夽
Dedicated to Professor James R. Durig on the occasion of his 65th birthday. * Corresponding author. Tel: ⫹ 1-901-678-4427; Fax: ⫹ 1-901678-3447. E-mail address: [email protected] (Y.-S. Li).
2. Experimental The sample of 2,2,2-trichloroethyl methyl ether
0022-2860/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(00)00514-7
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purified by using low temperature sublimation column. The microwave spectra were recorded in the frequency region 26.5–40.0 GHz, using a conventional microwave spectrometer [6] with a stark modulation frequency of 100 KHz. A microwave signal source was generated from a Hewlett-Packard 8457A microwave synthesizer. The microwave spectrum was recorded while the Stark cell cooled to near dry ice temperature (⬃⫺50⬚C). 3. Microwave spectra and results Fig. 1. The low-resolution microwave spectrum of 2,2,2-trichloroethyl methyl ether. The band markers are calculated from J ⫹ 1 with B ⫹ C 2065:4 MHz for the gauche conformer (G) and 1996.5 MHz for the trans conformer (T).
used in present study was prepared by the reaction of 2,2,2-trichloroethanol with dimethyl sulfate (both from Aldrich) along with the adding of potassium hydroxide while reacting. The reaction flask was fitted with condenser and was cooled with an ice bath. This mixture was then extracted with methylene chloride, and washed with dilute HCl followed by distilled water. The sample was also prepared by the reaction of dry sodium hydride, methyl iodide (Aldrich), and 2,2,2-trichloroethanol in dry ether under nitrogen at room temperature. The desired sample was separated by fractional distillation. The purity of the sample was checked by proton NMR and GC-mass spectrometer. Before collecting the infrared spectra, the sample was
The microwave spectrum of 2,2,2-trichloroethyl methyl ether is shown in Fig. 1. Two series of bands are observed in the spectrum; a more intense, broader series and a weaker, sharper series. These two series mostly likely arise from two non-equivalent conformers. In each series, six groups of bands appear to have nearly equal spacing between them. On the lower frequency side of each major band, there are several weak bands. These weak bands may arise from the natural chlorine isotopes 35Cl/ 37Cl, which are present in 75% and 25%, respectively. In considering the presence of low frequency vibrations in the molecule [5], some of these weak bands may arise from the most abundant molecules in excited vibration states. The character of equal spacing is typical of a nearly prolate rotor with a-type transitions or a nearly oblate rotor with c-type transitions. In order to determine the type of transitions for the observed brands in the series, the rotational constants of 2,2,2-trichloroethyl
Table 1 Calculated rotational constants (MHz) with modified C–C–O and C–O–C angles for 2,2,2-trichloroethyl methyl ether, C 35Cl3CH2OCH, in different conformations (different conformers as resulting from the internal rotation of the methoxy group around the C–O bond) Orientation angles a (u ) (degree)
0 30 60 90 120 150 180 a
Rotational constants A
B
C
⫺k
B⫹C
2A ⫺ B ⫺ C
1875.8 1878.2 1864.0 1837.3 1807.0 1783.5 1776.5
1134.8 1155.6 1138.9 1102.2 1056.8 1021.1 1013.2
1064.6 1074.5 1050.3 1018.4 990.8 978.7 984.9
0.8269 0.7983 0.7822 0.7952 0.8382 0.8946 0.9286
2199.4 2230.1 2189.2 2120.6 2047.6 1999.8 1998.1
1552.2 1526.3 1538.8 1554.0 1566.4 1567.3 1554.9
Here u 0⬚ if the methoxy group eclipses the C–C bond, i.e. cis conformer.
B.-Y. Liu et al. / Journal of Molecular Structure 550–551 (2000) 99–104 Table 2 Rotational transitions (MHz) of 2,2,2-trichloroethyl methyl ether, C 35Cl3CH2OCH3 Conformer
Transition
n (obsd.)
n (calc.) a
(B ⫹ C) b
Gauche
13 ← 12 14 ← 13 15 ← 14 16 ← 15 17 ← 16 18 ← 17 14 ← 13 15 ← 14 16 ← 15 17 ← 16 18 ← 17 19 ← 18
26857 28920 30985 33047 35110 37168 27954 29946 31947 33940 35935 37929
26851 28917 30982 33048 35113 37179 27951 29947 31944 33940 35937 27933
2065.9 2065.7 2065.7 2065.4 2065.3 2064.9 1996.7 1996.4 1996.7 1996.5 1996.4 1996.3
Trans
Calculated from the experimental mean value of B ⫹ C (2065.4 MHz for the gauche conformer and 1996.5 MHz for the trans conformer. b Calculated from the observed frequency in MHz. a
methyl ether were calculated by assuming a combination of the corresponding structural parameters of CC13CH3 [7], CH3CH2OCH3 [8], and CCl3CD3 [9]. In the calculation, the methyl group was assumed to be symmetric with respect to the O–C bond, and the CH3O– group was rotated with respect the O–C bond in an increment of 30⬚. The results of the calculation are listed in Table 1. It should be pointed out that the inclusion of the increment of 2.2⬚ and 2.6⬚ for the angle C–C–O and the angle C–O–C, respectively, for each 30⬚ of the OCH3 group internal rotation from the trans position results in a better fitting with the experimental values. In considering a more steric interaction in the gauche form than in the trans form, the opening up of the angles is reasonable. A zero
101
degree orientation represents an eclipsed conformation in which the O–CH3 bond eclipses the C–CCl3 bond in the Newman projection. From the results of the calculation, it is seen that the value of k varies from ⫺0.78 to ⫺0.93 with the angles of internal rotation. It is therefore concluded that the molecule must be a nearly prolate rotor and the six groups of bands observed for both series are due to a-type transitions. Also from Table 1 it should be noted that the values of the rotational constants are very sensitive to the angle of the internal rotation of the methoxy group. Since there might be more than one possible conformer for the CCl3CH2OCH3 molecule, analyzing the microwave spectrum should be able to give a conclusive identification to the abundant conformers of the molecule in the gas phase. For a nearly prolate rotor, a-type R-branch transitions J ⫹ 1 ← J may be expressed approximately by nJ J ⫹ 1 B ⫹ C: From the expression, the separation between the groups in the same series should be equal to B ⫹ C for the R-branch bands. Knowing separations along with the observed frequencies should provide sufficient information to assign the rotational bands. For this reason, the six bands of the most intense series observed at 26857, 28920, 30985, 33047, 35110 and 37168 MHz are assigned to 13 ← 12; 14 ← 13; 15 ← 14; 16 ← 15; 17 ← 16 and 18 ← 17; respectively (see Table 2). These assignments of the ground vubrational state transitions yield 2065.4 MHz for the mean value of B ⫹ C: In comparing the experimental value of B ⫹ C with the calculated B ⫹ C listed in Table 1, it is concluded that the six bands arise from the molecule in the nearly gauche form 120⬚ ⬎ 0 ⬎ 90⬚: A comparison of the experimental value of B ⫹ C
Table 3 Observed rotational transition frequencies (MHz) and B ⫹ C (MHz) for different species of gauche-2,2,2-trichloroethyl methyl ether Transition
18 ← 17 17 ← 16 16 ← 15 15 ← 14 14 ← 13 Average a
Species A ) a
Species B
Species C
Species D
Freq.
B⫹C
Freq.
B⫹C
Freq.
B⫹C
Freq.
B⫹C
37168 35110 33047 30985 28920
2064.9 2065.3 2065.4 2065.7 2065.7 2065.4
36952 34889 32836 30788 28740
2052.9 2052.3 2052.3 2052.5 2052.8 2052.6
36588 34559 32537 30498 28473
2032.7 2032.9 2033.6 2033.2 2033.8 2033.2
36378 34363 32328 30304 28297
2021.0 2021.4 2020.5 2020.3 2021.2 2020.9
Species A stands for the C 35Cl3CH2OCH3 isotopic species.
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Table 4 Observed rotational transition frequencies (MHz) for different species of trans-2,2,2-trichloroethyl methyl ether Transition
19 ← 18 18 ← 17 17 ← 16 16 ← 15 15 ← 14 14 ← 13 Mean a
Species A a
Species B
Species C
Freq. (MHz)
B⫹C
Freq. (MHz)
B⫹C
Freq. (MHz)
B⫹C
37929 35935 33940 31947 29946 27954
1996.3 1996.4 1996.5 1996.7 1996.4 1996.7 1996.5
37687 35700 33720 31740 29754 27770
1983.5 1983.3 1983.5 1983.8 1983.6 1983.6 1983.6
37408 35433 33464 31498 29530
1968.8 1968.5 1968.5 1968.6 1968.7 1968.6
Species A stands for the C 35Cl3CH2OCH3 isotopic species.
with the calculated ones at different angle (u ) suggested that the dihedral angle was ⬃110⬚. An advanced refinement was made based on the results of the MNDO calculation, indicating a slight internal rotation (10⬚) of CC13 group from the normal staggered position was preferred. The refined structure gives a B ⫹ C value of 2056.5 MHz, which is in reasonable agreement with the experimental value. The six other rather weak and sharp series observed at 27954, 29946, 31947, 33940, 35935 and 37929 MHz are assigned to 14 ← 13; 15 ← 14; 16 ←
15; 17 ← 16; 18 ← 17; and 19 ← 18; respectively (see Table 2). These rotational assignments of the ground state transitions give a mean value of 1996.5 MHz for the B ⫹ C: A comparison of the experimental value of B ⫹ C with the calculated B ⫹ C listed in Table 1, suggests that these bands must result from the molecule in the trans form u 180⬚: Tables 3 and 4 show the experimental results of the gauche and trans conformers, respectively. In order to obtain the characteristic information for chlorine isotopic species in the gauche and trans conformations,
Fig. 2. Experimental band markers for the trans (T) and gauche (G) conformers and for different species; predicted band markers for different isotopic species in the R-band frequency region. 37Cl(t) indicates that the 37Cl atom is trans to the oxygen atom; 37Cl(g): the 37Cl is gauche to the oxygen atom; 37Cl(g 0 ): the 37Cl is gauche to the oxygen atom and away from the methyl group.
B.-Y. Liu et al. / Journal of Molecular Structure 550–551 (2000) 99–104 Table 5 Calculated dipole moments (debye) of 2,2,2-trichloroethyl methyl ether Dipole moment
ma mb mc mt
CNDO trans
gauche
Vector model trans gauche
1.74 0.00 2.66 3.18
1.26 1.47 1.64 2.53
1.63 0.00 2.19 2.73
1.24 1.09 1.55 2.26
the rotational constants were calculated based on the assumed structural parameters, including the modified angles C–O–C and C–C–O. For the gauche conformer of 35Cl3 –, 37Cl(g) 35Cl2 –, 37Cl(g 0 ) 35Cl2 – and 37 Cl(t) 35Cl2CCH2OCH3 isotopic species, the values of B ⫹ C were calculated to be 2065.50, 2054.05, 2049.83 and 2033.72 MHz, respectively. For the trans 35 37 Cl3 –, Cl(g) 35Cl2 – and conformer of 37 35 Cl(t) Cl2CCH2OCH3, the values of B ⫹ C were calculated to be 1996.50, 1983.77 and 1967.56 MHz, respectively. The relative position of 37Cl in each of the species is given in the footnote of Fig. 2. Based on the results, the rotational frequencies for different isotopic species in the gauche and trans conformers are marked in Fig. 2 along with the observed rotational transition frequencies. A comparison of the predicted and the observed frequencies for the gauche conformer indicates the species A, B and C shown in Table 3 belong to the 35Cl3 –, 37Cl(g) 35Cl2 – and 37 Cl(t) 35Cl2CCH2OCH3 isotopic species, respectively. The species D may arise from molecules in the first excited vibration state. For molecules in the trans form, the species A, B and C shown in Table 4 must arise from the 35Cl3 –, 37Cl(g) 35Cl2 – and 37 Cl(t) 35Cl2CCH2OCH3 isotopic species, respectively. Spectrum arising from any other isotopic species could not be identified in the present study. To determine the stable conformer in gas phase, the dipole moment components were calculated based on the assumption that the total dipole moment of the molecule is the vector sum for the dipoles of the CCl3CH3 [7] and CH3OCH3 [10]. The orientation of the dipole vector in the –CH2OCH3 part was chosen bisecting the C–O–C plane. The dipole moment components are also calculated by the CNDO method. Data obtained from these two methods both indicate that the
103
trans form is more polar than the gauche form (Table 5). These calculations will provide useful information in determining the relative stability of the conformers.
4. Discussion From the investigation of the low-resolution microwave spectra of 2,2,2-trichloroethyl methyl ether, two conformers, trans and gauche, have been identified in the gas phase. The bands assigned to the trans form are about one-third less intense than the corresponding ones assigned to the gauche form. In order to estimate the relative abundance of these two conformers in the gas phase from the relative intensity data in the low-resolution microwave spectrum, the dipole moment components of the trans and gauche conformers were calculated with the molecular orbital program CNDO and the vector model. The results of both calculations are listed in Table 5. It is clearly shown that the trans from has a larger dipole moment component, m a, than the gauche form. From the dipole moment component m a along with the relative intensity, the energy difference between these two conformers in the gas phase was determined to be 56 cm ⫺1 (159 cal/mol) with the gauche form being more stable than the trans form. In studying the conformation of 2,2,2-trichloroethanol by vibrational spectroscopy [11], Perttila explained that the intramolecular interactions between –O–H and Cl stabilize the gauche conformer. He concluded that the gauche 2,2,2-trihaloethanol [11,12] is more stable than the trans form. However, upon the substitution of the hydroxyl hydrogen by a methyl group, such a hydrogen bonding should be no longer existent to any sufficient extent or certainly not predominate. It is possible to have a gauche configuration if there is little steric repulsion present when the hydroxyl proton was substituted by a methyl group. From the structure derived from the low-resolution microwave spectroscopic study, it was found that the closest distance between the chlorine of the CCl3 group and the hydrogen of the methyl group is larger than the sum of their van der Waals radii. Thus, it is possible to have gauche form as a stable conformer. In considering the small experimental energy difference between the trans and the gauche
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conformers in gas, both conformers may exist in a wide temperature range with comparable abundance. Nevertheless, from the relative intensity measurements of vibrational bands for CCl3CH2OCH3 at different temperatures, the energy difference between the conformers in the liquid phase has been determined to be 161 cm ⫺1 with the trans form being more stable [5]. This change of conformational stability with phase may be rationalized on the basis of the large dipole moment for the trans conformer compared to that of the gauche form (see Table 5). The large dipole moment of the trans form is expected to have a greater intermolecular interaction in the condensed phase than in the vapor phase; such interaction will stabilize the trans conformer relative to the other conformer. How much the relative energy that the stable conformer is lowered upon the changing from the gas to the condensed phase is dependent on the difference of the electric dipole moment. From the present results along with those obtained in the previous vibrational work [5], the energy of stabilization in referring to the gauche form upon changing from the gas to the liquid and crystalline states is 217 cm ⫺1 for the trans conformer. This result is in good agreement with 2,2,2-trifluoroethyl methyl ether [13], which lowers its trans conformational energy relative to that of the gauche form by about 192 cm ⫺1 upon the condensation. The energy difference between the two conformers of CCl3CH2OCH3 is rather small compared to those of 2,2,2-trifluoroethyl methyl ether [13] and ethyl methyl ether [14–16]. According to the results obtained by Matsuura et al. from their vibrational study [17], the rather polar TG form of 2-chloroethyl methyl ether is the only conformer identified in crystalline. Additionally, the TG form is more favorable in the liquid than in the
gas; the abundance of the less polar GT conformer becomes more favorable in going from the liquid to the gas. All these indicate the importance of the dipole–dipole intermolecular interactions in the condensed phase.
References [1] J.F. Reinhard, M.N. Lewis, R.J. Meltzer, J.A. King, J. Am. Pharm. Assoc. 46 (1957) 359. [2] K. Okazaki, S. Nagase, H. Baba, K. Kodaira, J. Fluorine Chem. 4 (1974) 387. [3] J.S. Moilliet, K. Pearson, R.W. Rendell,, Ger. Offen. 2554884 (C1. C07C43/12), 16 June 1976; Brit. Appl. 74/52834, 06 December 1974, 14pp. [4] D. Zanke, E. Kluge, L. Banasiak, Tagungsber-Akad. Landwirtschaftswiss DDR 222 (1984) 277. [5] Y.-S. Li, B.-Y. Lin, Y.-S. Cai, J. Lin, Y. Li, Y.-T. Wang, Acta Chim. Sinica 49 (1991) 797. [6] K.M. McAfee, R.H. Hughes, E.B. Wilson Jr, Rev. Sci. Instrum. 20 (1949) 821. [7] R. Holm, M. Mitzlaff, H. Hartmann, Z. Naturforsch. 23A (1968) 307. [8] M. Hayashi, K. Kuwada, J. Mol. Struct. 29 (1975) 147. [9] J.R. Durig, M.M. Chen, Y.S. Li, J. Mol. Struct. 15 (1973) 37. [10] R.D. Nelson Jr., D.R. Lide Jr., A.A. Maryotte, Selected values of electric dipole moments for molecules in the gas phase, National Bureau of Standard NSRDS-NBS10, Govt. Printing Office, Washington DC, 1967. [11] M. Perttila, Spectrochim. Acta 35A (1978) 585. [12] V.F. Kalasinsky, H.V. Anjaria, J. Phys. Chem. 84 (1980) 1940. [13] Y.S. Li, F.O. Cox, J.R. Durig, J. Phys. Chem. 91 (1987) 1344. [14] T. Kitagawa, T. Miyazawa, Bull. Chem. Soc. Jpn 41 (1968) 1976. [15] J.P. Perchard, Spectrochim. Acta, Part A 26 (1970) 707. [16] T. Kitagawa, K. Kusaki, T. Miyazawa, Bull. Chem. Soc. Jpn 46 (1973) 3685. [17] H. Matsuura, M. Kono, H. Iizuka, Y. Ogawa, I. Harada, T. Shimanouchi, Bull. Chem. Soc. Jpn 50 (1977) 2272.
Conformational analysis of 2,2,2-trichloroethyl methyl ether by low-resolution microwave spectroscopy 夽 B.-Y. Liu, H. Mohamad, Y.-S. Li* The University of Memphis, Department of Chemistry, Memphis, TN 38152, USA Received 6 July 1999; accepted 9 November 1999
Abstract The low-resolution microwave spectra of 2,2,2-trichloroethyl methyl ether have been recorded from 26.5 to 39.0 GHz. From the spacing of the major bands, it is shown that the value of 2065.4 MHz for B ⫹ C is consistent with the presence of the gauche conformer. Another set of bands of approximately two-third the intensity of the major bands has also been observed. The value of 1996.5 MHz for B ⫹ C is consistent with the presence of the trans conformer. The microwave data indicate that the gauche conformer is the abundant one in the gas phase. Interpretation as to the dominance of the gauche conformer will be suggested. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Conformational analysis; 2,2,2-Trichloroethyl methyl ether; Microwave spectrum; Low resolution microwave spectrum
1. Introduction The 2,2,2-trichloroethyl methyl was first prepared in 1951 [1]. It was then synthesized as anesthetics [2,3]. Zanke et al. [4] reported that the compound has preventive and curative activities against Phytophthora infestants on tomato leaf disks. In an earlier study, we recorded the infrared and Raman spectra in the frequency region 4000–35 cm ⫺1, carried out a normal coordinate analysis, and assigned the vibrational modes for the molecule [5]. Additionally, we have identified two non-equivalent conformers in the study. From the variation of the relative intensities of the CCO bending vibrations with temperature in liquid, the enthalpy difference was
determined to be 116 ^ 11 cm ⫺1 with the trans conformer being more stable. In the present study, we have collected the lowresolution microwave spectrum of 2,2,2-trichloroethyl methyl ether. The basic information obtained from the analysis of the experimental rotational transition frequencies includes the molecular rotational constants, which are inversely proportional to the principal moments of inertia. The principal moments of inertia are closely related to the molecular structure. In general, non-equivalent rotational isomers have different principal moments of inertia and hence distinctly different rotational spectra. By comparing the calculated rotational constants to the observed ones, the conformation of a molecule can be identified.
夽
Dedicated to Professor James R. Durig on the occasion of his 65th birthday. * Corresponding author. Tel: ⫹ 1-901-678-4427; Fax: ⫹ 1-901678-3447. E-mail address: [email protected] (Y.-S. Li).
2. Experimental The sample of 2,2,2-trichloroethyl methyl ether
0022-2860/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(00)00514-7
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B.-Y. Liu et al. / Journal of Molecular Structure 550–551 (2000) 99–104
purified by using low temperature sublimation column. The microwave spectra were recorded in the frequency region 26.5–40.0 GHz, using a conventional microwave spectrometer [6] with a stark modulation frequency of 100 KHz. A microwave signal source was generated from a Hewlett-Packard 8457A microwave synthesizer. The microwave spectrum was recorded while the Stark cell cooled to near dry ice temperature (⬃⫺50⬚C). 3. Microwave spectra and results Fig. 1. The low-resolution microwave spectrum of 2,2,2-trichloroethyl methyl ether. The band markers are calculated from J ⫹ 1 with B ⫹ C 2065:4 MHz for the gauche conformer (G) and 1996.5 MHz for the trans conformer (T).
used in present study was prepared by the reaction of 2,2,2-trichloroethanol with dimethyl sulfate (both from Aldrich) along with the adding of potassium hydroxide while reacting. The reaction flask was fitted with condenser and was cooled with an ice bath. This mixture was then extracted with methylene chloride, and washed with dilute HCl followed by distilled water. The sample was also prepared by the reaction of dry sodium hydride, methyl iodide (Aldrich), and 2,2,2-trichloroethanol in dry ether under nitrogen at room temperature. The desired sample was separated by fractional distillation. The purity of the sample was checked by proton NMR and GC-mass spectrometer. Before collecting the infrared spectra, the sample was
The microwave spectrum of 2,2,2-trichloroethyl methyl ether is shown in Fig. 1. Two series of bands are observed in the spectrum; a more intense, broader series and a weaker, sharper series. These two series mostly likely arise from two non-equivalent conformers. In each series, six groups of bands appear to have nearly equal spacing between them. On the lower frequency side of each major band, there are several weak bands. These weak bands may arise from the natural chlorine isotopes 35Cl/ 37Cl, which are present in 75% and 25%, respectively. In considering the presence of low frequency vibrations in the molecule [5], some of these weak bands may arise from the most abundant molecules in excited vibration states. The character of equal spacing is typical of a nearly prolate rotor with a-type transitions or a nearly oblate rotor with c-type transitions. In order to determine the type of transitions for the observed brands in the series, the rotational constants of 2,2,2-trichloroethyl
Table 1 Calculated rotational constants (MHz) with modified C–C–O and C–O–C angles for 2,2,2-trichloroethyl methyl ether, C 35Cl3CH2OCH, in different conformations (different conformers as resulting from the internal rotation of the methoxy group around the C–O bond) Orientation angles a (u ) (degree)
0 30 60 90 120 150 180 a
Rotational constants A
B
C
⫺k
B⫹C
2A ⫺ B ⫺ C
1875.8 1878.2 1864.0 1837.3 1807.0 1783.5 1776.5
1134.8 1155.6 1138.9 1102.2 1056.8 1021.1 1013.2
1064.6 1074.5 1050.3 1018.4 990.8 978.7 984.9
0.8269 0.7983 0.7822 0.7952 0.8382 0.8946 0.9286
2199.4 2230.1 2189.2 2120.6 2047.6 1999.8 1998.1
1552.2 1526.3 1538.8 1554.0 1566.4 1567.3 1554.9
Here u 0⬚ if the methoxy group eclipses the C–C bond, i.e. cis conformer.
B.-Y. Liu et al. / Journal of Molecular Structure 550–551 (2000) 99–104 Table 2 Rotational transitions (MHz) of 2,2,2-trichloroethyl methyl ether, C 35Cl3CH2OCH3 Conformer
Transition
n (obsd.)
n (calc.) a
(B ⫹ C) b
Gauche
13 ← 12 14 ← 13 15 ← 14 16 ← 15 17 ← 16 18 ← 17 14 ← 13 15 ← 14 16 ← 15 17 ← 16 18 ← 17 19 ← 18
26857 28920 30985 33047 35110 37168 27954 29946 31947 33940 35935 37929
26851 28917 30982 33048 35113 37179 27951 29947 31944 33940 35937 27933
2065.9 2065.7 2065.7 2065.4 2065.3 2064.9 1996.7 1996.4 1996.7 1996.5 1996.4 1996.3
Trans
Calculated from the experimental mean value of B ⫹ C (2065.4 MHz for the gauche conformer and 1996.5 MHz for the trans conformer. b Calculated from the observed frequency in MHz. a
methyl ether were calculated by assuming a combination of the corresponding structural parameters of CC13CH3 [7], CH3CH2OCH3 [8], and CCl3CD3 [9]. In the calculation, the methyl group was assumed to be symmetric with respect to the O–C bond, and the CH3O– group was rotated with respect the O–C bond in an increment of 30⬚. The results of the calculation are listed in Table 1. It should be pointed out that the inclusion of the increment of 2.2⬚ and 2.6⬚ for the angle C–C–O and the angle C–O–C, respectively, for each 30⬚ of the OCH3 group internal rotation from the trans position results in a better fitting with the experimental values. In considering a more steric interaction in the gauche form than in the trans form, the opening up of the angles is reasonable. A zero
101
degree orientation represents an eclipsed conformation in which the O–CH3 bond eclipses the C–CCl3 bond in the Newman projection. From the results of the calculation, it is seen that the value of k varies from ⫺0.78 to ⫺0.93 with the angles of internal rotation. It is therefore concluded that the molecule must be a nearly prolate rotor and the six groups of bands observed for both series are due to a-type transitions. Also from Table 1 it should be noted that the values of the rotational constants are very sensitive to the angle of the internal rotation of the methoxy group. Since there might be more than one possible conformer for the CCl3CH2OCH3 molecule, analyzing the microwave spectrum should be able to give a conclusive identification to the abundant conformers of the molecule in the gas phase. For a nearly prolate rotor, a-type R-branch transitions J ⫹ 1 ← J may be expressed approximately by nJ J ⫹ 1 B ⫹ C: From the expression, the separation between the groups in the same series should be equal to B ⫹ C for the R-branch bands. Knowing separations along with the observed frequencies should provide sufficient information to assign the rotational bands. For this reason, the six bands of the most intense series observed at 26857, 28920, 30985, 33047, 35110 and 37168 MHz are assigned to 13 ← 12; 14 ← 13; 15 ← 14; 16 ← 15; 17 ← 16 and 18 ← 17; respectively (see Table 2). These assignments of the ground vubrational state transitions yield 2065.4 MHz for the mean value of B ⫹ C: In comparing the experimental value of B ⫹ C with the calculated B ⫹ C listed in Table 1, it is concluded that the six bands arise from the molecule in the nearly gauche form 120⬚ ⬎ 0 ⬎ 90⬚: A comparison of the experimental value of B ⫹ C
Table 3 Observed rotational transition frequencies (MHz) and B ⫹ C (MHz) for different species of gauche-2,2,2-trichloroethyl methyl ether Transition
18 ← 17 17 ← 16 16 ← 15 15 ← 14 14 ← 13 Average a
Species A ) a
Species B
Species C
Species D
Freq.
B⫹C
Freq.
B⫹C
Freq.
B⫹C
Freq.
B⫹C
37168 35110 33047 30985 28920
2064.9 2065.3 2065.4 2065.7 2065.7 2065.4
36952 34889 32836 30788 28740
2052.9 2052.3 2052.3 2052.5 2052.8 2052.6
36588 34559 32537 30498 28473
2032.7 2032.9 2033.6 2033.2 2033.8 2033.2
36378 34363 32328 30304 28297
2021.0 2021.4 2020.5 2020.3 2021.2 2020.9
Species A stands for the C 35Cl3CH2OCH3 isotopic species.
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Table 4 Observed rotational transition frequencies (MHz) for different species of trans-2,2,2-trichloroethyl methyl ether Transition
19 ← 18 18 ← 17 17 ← 16 16 ← 15 15 ← 14 14 ← 13 Mean a
Species A a
Species B
Species C
Freq. (MHz)
B⫹C
Freq. (MHz)
B⫹C
Freq. (MHz)
B⫹C
37929 35935 33940 31947 29946 27954
1996.3 1996.4 1996.5 1996.7 1996.4 1996.7 1996.5
37687 35700 33720 31740 29754 27770
1983.5 1983.3 1983.5 1983.8 1983.6 1983.6 1983.6
37408 35433 33464 31498 29530
1968.8 1968.5 1968.5 1968.6 1968.7 1968.6
Species A stands for the C 35Cl3CH2OCH3 isotopic species.
with the calculated ones at different angle (u ) suggested that the dihedral angle was ⬃110⬚. An advanced refinement was made based on the results of the MNDO calculation, indicating a slight internal rotation (10⬚) of CC13 group from the normal staggered position was preferred. The refined structure gives a B ⫹ C value of 2056.5 MHz, which is in reasonable agreement with the experimental value. The six other rather weak and sharp series observed at 27954, 29946, 31947, 33940, 35935 and 37929 MHz are assigned to 14 ← 13; 15 ← 14; 16 ←
15; 17 ← 16; 18 ← 17; and 19 ← 18; respectively (see Table 2). These rotational assignments of the ground state transitions give a mean value of 1996.5 MHz for the B ⫹ C: A comparison of the experimental value of B ⫹ C with the calculated B ⫹ C listed in Table 1, suggests that these bands must result from the molecule in the trans form u 180⬚: Tables 3 and 4 show the experimental results of the gauche and trans conformers, respectively. In order to obtain the characteristic information for chlorine isotopic species in the gauche and trans conformations,
Fig. 2. Experimental band markers for the trans (T) and gauche (G) conformers and for different species; predicted band markers for different isotopic species in the R-band frequency region. 37Cl(t) indicates that the 37Cl atom is trans to the oxygen atom; 37Cl(g): the 37Cl is gauche to the oxygen atom; 37Cl(g 0 ): the 37Cl is gauche to the oxygen atom and away from the methyl group.
B.-Y. Liu et al. / Journal of Molecular Structure 550–551 (2000) 99–104 Table 5 Calculated dipole moments (debye) of 2,2,2-trichloroethyl methyl ether Dipole moment
ma mb mc mt
CNDO trans
gauche
Vector model trans gauche
1.74 0.00 2.66 3.18
1.26 1.47 1.64 2.53
1.63 0.00 2.19 2.73
1.24 1.09 1.55 2.26
the rotational constants were calculated based on the assumed structural parameters, including the modified angles C–O–C and C–C–O. For the gauche conformer of 35Cl3 –, 37Cl(g) 35Cl2 –, 37Cl(g 0 ) 35Cl2 – and 37 Cl(t) 35Cl2CCH2OCH3 isotopic species, the values of B ⫹ C were calculated to be 2065.50, 2054.05, 2049.83 and 2033.72 MHz, respectively. For the trans 35 37 Cl3 –, Cl(g) 35Cl2 – and conformer of 37 35 Cl(t) Cl2CCH2OCH3, the values of B ⫹ C were calculated to be 1996.50, 1983.77 and 1967.56 MHz, respectively. The relative position of 37Cl in each of the species is given in the footnote of Fig. 2. Based on the results, the rotational frequencies for different isotopic species in the gauche and trans conformers are marked in Fig. 2 along with the observed rotational transition frequencies. A comparison of the predicted and the observed frequencies for the gauche conformer indicates the species A, B and C shown in Table 3 belong to the 35Cl3 –, 37Cl(g) 35Cl2 – and 37 Cl(t) 35Cl2CCH2OCH3 isotopic species, respectively. The species D may arise from molecules in the first excited vibration state. For molecules in the trans form, the species A, B and C shown in Table 4 must arise from the 35Cl3 –, 37Cl(g) 35Cl2 – and 37 Cl(t) 35Cl2CCH2OCH3 isotopic species, respectively. Spectrum arising from any other isotopic species could not be identified in the present study. To determine the stable conformer in gas phase, the dipole moment components were calculated based on the assumption that the total dipole moment of the molecule is the vector sum for the dipoles of the CCl3CH3 [7] and CH3OCH3 [10]. The orientation of the dipole vector in the –CH2OCH3 part was chosen bisecting the C–O–C plane. The dipole moment components are also calculated by the CNDO method. Data obtained from these two methods both indicate that the
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trans form is more polar than the gauche form (Table 5). These calculations will provide useful information in determining the relative stability of the conformers.
4. Discussion From the investigation of the low-resolution microwave spectra of 2,2,2-trichloroethyl methyl ether, two conformers, trans and gauche, have been identified in the gas phase. The bands assigned to the trans form are about one-third less intense than the corresponding ones assigned to the gauche form. In order to estimate the relative abundance of these two conformers in the gas phase from the relative intensity data in the low-resolution microwave spectrum, the dipole moment components of the trans and gauche conformers were calculated with the molecular orbital program CNDO and the vector model. The results of both calculations are listed in Table 5. It is clearly shown that the trans from has a larger dipole moment component, m a, than the gauche form. From the dipole moment component m a along with the relative intensity, the energy difference between these two conformers in the gas phase was determined to be 56 cm ⫺1 (159 cal/mol) with the gauche form being more stable than the trans form. In studying the conformation of 2,2,2-trichloroethanol by vibrational spectroscopy [11], Perttila explained that the intramolecular interactions between –O–H and Cl stabilize the gauche conformer. He concluded that the gauche 2,2,2-trihaloethanol [11,12] is more stable than the trans form. However, upon the substitution of the hydroxyl hydrogen by a methyl group, such a hydrogen bonding should be no longer existent to any sufficient extent or certainly not predominate. It is possible to have a gauche configuration if there is little steric repulsion present when the hydroxyl proton was substituted by a methyl group. From the structure derived from the low-resolution microwave spectroscopic study, it was found that the closest distance between the chlorine of the CCl3 group and the hydrogen of the methyl group is larger than the sum of their van der Waals radii. Thus, it is possible to have gauche form as a stable conformer. In considering the small experimental energy difference between the trans and the gauche
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conformers in gas, both conformers may exist in a wide temperature range with comparable abundance. Nevertheless, from the relative intensity measurements of vibrational bands for CCl3CH2OCH3 at different temperatures, the energy difference between the conformers in the liquid phase has been determined to be 161 cm ⫺1 with the trans form being more stable [5]. This change of conformational stability with phase may be rationalized on the basis of the large dipole moment for the trans conformer compared to that of the gauche form (see Table 5). The large dipole moment of the trans form is expected to have a greater intermolecular interaction in the condensed phase than in the vapor phase; such interaction will stabilize the trans conformer relative to the other conformer. How much the relative energy that the stable conformer is lowered upon the changing from the gas to the condensed phase is dependent on the difference of the electric dipole moment. From the present results along with those obtained in the previous vibrational work [5], the energy of stabilization in referring to the gauche form upon changing from the gas to the liquid and crystalline states is 217 cm ⫺1 for the trans conformer. This result is in good agreement with 2,2,2-trifluoroethyl methyl ether [13], which lowers its trans conformational energy relative to that of the gauche form by about 192 cm ⫺1 upon the condensation. The energy difference between the two conformers of CCl3CH2OCH3 is rather small compared to those of 2,2,2-trifluoroethyl methyl ether [13] and ethyl methyl ether [14–16]. According to the results obtained by Matsuura et al. from their vibrational study [17], the rather polar TG form of 2-chloroethyl methyl ether is the only conformer identified in crystalline. Additionally, the TG form is more favorable in the liquid than in the
gas; the abundance of the less polar GT conformer becomes more favorable in going from the liquid to the gas. All these indicate the importance of the dipole–dipole intermolecular interactions in the condensed phase.
References [1] J.F. Reinhard, M.N. Lewis, R.J. Meltzer, J.A. King, J. Am. Pharm. Assoc. 46 (1957) 359. [2] K. Okazaki, S. Nagase, H. Baba, K. Kodaira, J. Fluorine Chem. 4 (1974) 387. [3] J.S. Moilliet, K. Pearson, R.W. Rendell,, Ger. Offen. 2554884 (C1. C07C43/12), 16 June 1976; Brit. Appl. 74/52834, 06 December 1974, 14pp. [4] D. Zanke, E. Kluge, L. Banasiak, Tagungsber-Akad. Landwirtschaftswiss DDR 222 (1984) 277. [5] Y.-S. Li, B.-Y. Lin, Y.-S. Cai, J. Lin, Y. Li, Y.-T. Wang, Acta Chim. Sinica 49 (1991) 797. [6] K.M. McAfee, R.H. Hughes, E.B. Wilson Jr, Rev. Sci. Instrum. 20 (1949) 821. [7] R. Holm, M. Mitzlaff, H. Hartmann, Z. Naturforsch. 23A (1968) 307. [8] M. Hayashi, K. Kuwada, J. Mol. Struct. 29 (1975) 147. [9] J.R. Durig, M.M. Chen, Y.S. Li, J. Mol. Struct. 15 (1973) 37. [10] R.D. Nelson Jr., D.R. Lide Jr., A.A. Maryotte, Selected values of electric dipole moments for molecules in the gas phase, National Bureau of Standard NSRDS-NBS10, Govt. Printing Office, Washington DC, 1967. [11] M. Perttila, Spectrochim. Acta 35A (1978) 585. [12] V.F. Kalasinsky, H.V. Anjaria, J. Phys. Chem. 84 (1980) 1940. [13] Y.S. Li, F.O. Cox, J.R. Durig, J. Phys. Chem. 91 (1987) 1344. [14] T. Kitagawa, T. Miyazawa, Bull. Chem. Soc. Jpn 41 (1968) 1976. [15] J.P. Perchard, Spectrochim. Acta, Part A 26 (1970) 707. [16] T. Kitagawa, K. Kusaki, T. Miyazawa, Bull. Chem. Soc. Jpn 46 (1973) 3685. [17] H. Matsuura, M. Kono, H. Iizuka, Y. Ogawa, I. Harada, T. Shimanouchi, Bull. Chem. Soc. Jpn 50 (1977) 2272.